Anisotropic phonon properties and effective electron mass in α -Ga2O3

Abstract

The ordinary and extraordinary infrared dielectric functions of α-Ga2O3 thin films with the corundum structure were investigated. The films were grown by mist chemical vapor epitaxy on (10 1 0) sapphire substrates in (10 1 0) surface orientation. They were doped by the donor tin with the resulting free-electron concentrations between 1017 and 1019 cm-3. Structural characterization revealed the absence of strain and the high crystallographic quality of the samples. It allows one to resolve all 7 Raman modes. 5 out of 6 infrared active transverse optical phonon modes lie in the investigated wave number range >250 cm-1, and are unambiguously identified by spectroscopic ellipsometry. For the highest doped sample, the free-carrier contribution (plasmon) to the infrared spectra allowed accurate determination of effective electron masses. They are m = (0.297 ± 0.010) m 0 (perpendicular to [0001]) and m | |∗ = (0.316 ± 0.007) m 0 (parallel to [0001]) for an electron concentration of n = 1.1 × 1019 cm-3.